Voltage regulator protected against short-circuits by current limiter responsive to output voltage

1. Field of the Invention

The present invention relates to the field of voltage regulators and in particular to regulators with a low drop out.

2. Description of the Related Art

A low drop out (LDO) regulator made in the form of an integrated circuit may be used to provide a predetermined voltage with low noise to a set of electronic circuits from a supply voltage provided by a rechargeable battery. Such a supply voltage decreases along time and is likely to include noise caused by the action of neighboring electromagnetic radiations on the battery-to-regulator connections. The regulator is said to have a low drop out since it enables providing a voltage close to the supply voltage.

FIG. 1 schematically shows a conventional low drop out regulator. The regulator includes an output terminal O provided to be connected to a load R. Load R, which is essentially resistive, represents the general input impedance of all the circuits supplied by the regulator. For simplicity, it is considered hereafter that load R is a resistor. The regulator includes an operational amplifier 2 having an inverting input E − connected to a positive reference voltage Vref and having a non-inverting inverting input E + connected to output terminal O by a feedback loop. Operational amplifier 2 is supplied between a positive supply voltage Vbat provided by the battery and a ground voltage GND. A P-channel MOS power transistor T 1 has its drain connected to output terminal O and its source connected to voltage Vbat. The gate of transistor T 1 is connected to the output terminal of inverting amplifier 2 . Transistor T 1 is a MOS transistor, especially to minimize, with respect to the use of a bipolar transistor, the difference between output voltage Vout of terminal O and supply voltage Vbat. A charge capacitor C is arranged between output terminal O and voltage GND.

The regulator maintains the voltage of output terminal O to a value equal to reference voltage Vref. Any variation in voltage Vbat translates as a variation in voltage Vout, which is transmitted by the feedback loop on terminal E − . Any variation in load R translates as a variation in current Iout provided by the regulator to the load. When load R decreases, current Iout increases. Conventionally, the voltage regulator includes a device of protection against short-circuits intended for limiting the regulator consumption by setting the maximum current that can be provided by the regulator.

The regulator includes a device 4 of protection against short-circuits. Device 4 includes a P-channel MOS transistor T 2 having its source connected to the gate of transistor T 1 . The drain of transistor T 2 is connected to the drain and to the gate of an N-channel MOS transistor 6 having its source connected to voltage GND. A current source CS generating a current Iref is also connected to the drain of transistor T 2 . An N-channel MOS transistor 7 has its source connected to voltage GND and its gate connected to the gate of transistor 6 . Transistor 7 is connected to voltage Vbat via a resistor R 1 . A P-channel MOS transistor T 3 has its source connected to voltage Vbat, its drain connected to the gate of transistor T 1 , and its gate connected to the drain of transistor 7 .

Current Irep flowing through transistor T 2 depends on current Iout flowing through transistor T 1 due to the fact that the sources of these transistors are interconnected and that their gates receive a same signal. The current flowing through transistor 6 is null when current Irep flowing through transistor T 2 is smaller than current Iref. No current then flows through transistor 7 and resistor R 1 , and the gate of transistor T 3 has a voltage equal to Vbat. When current Irep is greater than Iref, transistor 6 , transistor 7 , and resistor R 1 are run through by a current equal to Irep−Iref. The gate of transistor T 3 then has a potential equal to Vbat−R 1 (Irep−Iref). Transistors T 2 and T 3 , resistor R 1 , and current Iref are chosen so that, when current Iout is smaller than a threshold value It, transistor T 3 is not on. If current Iout exceeds threshold value It, transistor T 3 turns on and tends to bring the gate voltage of transistor T 1 to voltage Vbat. Transistor T 1 then becomes less conductive and current Iout returns to limiting value It. Circuit 4 thus enables limiting the current in the load to value It. Current It must be greater than the nominal current to be provided by the regulator.

A disadvantage of device 4 is that upon power-on of the regulator, capacitor C is charged with a current equal to current It whatever the value of resistance R. This high-current charge results in heating up and damaging capacitor C.

An object of the present invention is to provide a device of protection against short-circuits which enables avoiding for capacitor C to be run through by a strong current at the regulator power-on.

To achieve this object, the present invention provides a voltage regulator having an output terminal adapted to being connected to a load, including a device for limiting the current flowing through the load to a first threshold current if the voltage of the output terminal is lower than a threshold voltage, and to a second current threshold higher than the first current threshold if the voltage of the output terminal is greater than the threshold voltage.

According to an embodiment of the present invention, the limiting device includes a comparator for comparing the voltage of the output terminal to the threshold voltage, first and second feedback loops for limiting the current flowing through the load respectively to the first and second current thresholds, and a switching block controllable by the comparator to activate either the first or the second feedback loop according to whether the voltage of the output terminal is smaller or not than the threshold voltage.

According to an embodiment of the present invention, the switching block is adapted to providing a current depending on the current running through the load on a first or on a second output, and each feedback loop, connected to an output of the switching block, includes a control block adapted to providing a control signal when it receives from the switching block a current greater than a reference current, and further includes a turn-off means which receives the output of the control blocks and which decreases the current running through the load when any one of the first and second control signals is active.

According to an embodiment of the present invention, the voltage regulator includes a power switch arranged to connect the output terminal to a first supply voltage, and a first operational amplifier having its inverting and non-inverting inputs respectively connected to the reference voltage and to the output terminal, a control terminal of the power switch being connected to the output of the first operational amplifier and the device for limiting the current flowing through the load being connected to the control terminal of the power switch, the load including a capacitor and a first impedance connected in parallel between the output terminal and a second supply voltage.

According to an embodiment of the present invention, the switching block includes a first MOS transistor of a first type having its source connected to the first supply voltage and its gate connected to the control terminal of the power switch, and second and third MOS transistors of the first type having their sources connected to the drain of the first transistor, the drains of the second and third transistors respectively forming the first and second outputs of the switching block.

According to an embodiment of the present invention, the comparator includes fourth and fifth MOS transistors of a second type having their drains connected to the first supply voltage, having their gates respectively connected to the threshold voltage and to the output terminal, the sources of the fourth and fifth transistors being respectively connected to the gates of the second and third transistors, as well as to the second supply voltage via first and second current sources.

According to an embodiment of the present invention, the control block of each feedback loop includes a pair of MOS transistors of the second type having their sources connected to the second supply voltage, having their gates connected to each other and to a current source generating a reference current, the drain and the gate of a first transistor of the transistor pair being interconnected and connected to one of the outputs of the switching block, the current running through the second transistor of the transistor pair corresponding to the control signal provided by the control block.

According to an embodiment of the present invention, the turn-off means which receives the output of the control blocks includes a resistor having a first terminal connected to the first supply voltage and a second terminal arranged to receive the sum of the control signals provided by the control blocks, and a sixth MOS transistor of the first type having its source connected to the first supply voltage, having its drain connected to the control terminal of the power switch, and having its gate connected to the second terminal of the resistor.

According to an embodiment of the present invention, the turn-off means which receives the output of the control blocks includes a second impedance having a first terminal connected to the first supply voltage and a second terminal arranged to receive the sum of the control signals provided by the control blocks, a third impedance, matched with the second impedance, a first terminal of which is connected to the first supply voltage and a second terminal of which receives a predetermined constant current, a second operational amplifier having its non-inverting and inverting inputs respectively connected to the second terminal of the second and third impedances, and a seventh MOS transistor of the first type having its source connected to the first supply voltage, having its drain connected to the control terminal of the power switch, and having its gate connected to the output of the second operational amplifier.

According to an embodiment of the present invention, the first supply voltage, the reference voltage, and the threshold voltage are positive voltages of decreasing values, the second supply voltage is a ground voltage, the power switch and the transistors of the first type are P-channel MOS transistors, and the transistors of the second type are N-channel MOS transistors.

The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

FIG. 1, previously described, schematically shows a voltage regulator provided with a conventional device of protection against short-circuits;

FIG. 2 schematically shows a voltage regulator including a current-limiting device according to the present invention;

FIG. 3 schematically shows a first embodiment of the voltage regulator of FIG. 2;

FIG. 4 shows an example of forming of the voltage regulator of FIG. 3; and

FIG. 5 schematically shows a second embodiment of the voltage regulator of FIG. 2 .

FIG. 2 schematically shows a voltage regulator having an output terminal O connected to a load R, and which includes operational amplifier 2 , transistor T 1 and charge capacitor C of the previously-described conventional regulator. According to the present invention, the regulator includes a current-limiting device 8 having a first input terminal connected to output terminal O and a second input terminal connected to a threshold voltage Vt. Device 8 is further connected to the gate of transistor T 1 .

Device 8 compares voltage Vout of terminal O with voltage Vt. Voltage Vt is chosen to be smaller than voltage Vref. According to whether Vout is smaller or greater than Vt, current Iout is limited to a first or to a second low or high threshold current It 1 or It 2 . Upon regulator power-on, capacitor C is charged by current It 1 until voltage Vout reaches value Vt. Current It 1 is low to avoid damaging capacitor C. When voltage Vout becomes greater than voltage Vt, the current running through transistor T 1 becomes equal to It 2 while capacitor C is not completely charged. The end of the charge of capacitor C occurs with current It 2 . After the regulator powering-on, if load R becomes small without voltage Vout dropping below voltage Vt, for example in case of a limited short-circuit of load R, current Iout is limited to current It 2 . The current provided by the regulator then is substantially equal to the current provided by a regulator provided with a conventional protection device if It 2 =It. If load R becomes very small and voltage Vout drops below voltage Vt, for example, in case of a clear short-circuit, the current running through transistor T 1 is limited to current It 1 . The current provided by the regulator then is smaller than the current provided by a regulator provided with a conventional protection device, which is an additional advantage of the present invention.

FIG. 3 schematically shows a first embodiment of the voltage regulator of FIG. 2 . Device 8 includes a P-channel MOS transistor T 2 , having its source connected to voltage Vbat and its gate connected to the gate of transistor T 1 . Transistor T 2 is arranged to be run through by a current Irep depending on output current Iout. The drain of transistor T 2 is connected to an input terminal of a switching means 10 . A voltage comparator 12 has a first input terminal connected to output terminal O, a second input terminal connected to a threshold voltage Vt, and is provided to control switching means 10 . A first output terminal of switching means 10 is connected to an input terminal of a control means 14 , which controls a switch 16 , and a second output terminal of switching means 10 is connected to an input terminal of a control means 20 which controls a switch 22 . A P-channel MOS transistor T 3 has its source connected to voltage Vbat and its drain connected to the gate of transistor T 1 . The gate of transistor T 3 is coupled to a node G 3 . Node G 3 is connected to voltage Vbat via a resistor R 1 . Further, node G 3 is connected to voltage GND via switches 22 and 16 , in parallel.

Voltage comparator 12 controls switching means 10 so that current Irep is provided either to control means 14 , or to control means 20 , according to whether voltage Vout is smaller or greater than voltage Vt.

In the case where voltage Vout is smaller than voltage Vt, current Irep is provided to control means 14 . Control means 14 is provided to maintain switch 16 open or closed according to whether current Irep, received on its input terminal, is smaller or greater than a reference current Iref 1 . When current Irep becomes greater than current Iref 1 , switch 16 is closed and a current flows through resistor R 1 . The voltage of node G 3 drops, transistor T 3 turns on and decreases the conduction of transistors T 1 and T 2 until current Irep becomes smaller than current Iref 1 . The circuit acts as a current limiter limiting current Irep to value Iref 1 . Current Iout is thus limited to a current It 1 depending on current Iref 1 .

In the case where voltage Vout is greater than voltage Vt, switching means 10 is controlled by voltage comparator 12 so that current Irep is provided to the input terminal of control means 20 . Control means 20 , which has the same structure as control means 14 , is provided to maintain switch 22 off or on according to whether the current received on its input terminal is smaller or greater than a reference current Iref 2 . Current Iout provided by the voltage regulator is then limited to a value It 2 depending on current Iref 2 .

FIG. 4 shows an example of forming of the voltage regulator of FIG. 3 . Voltage comparator 12 includes two N-channel MOS transistors T 4 and T 5 , having their drains connected to voltage Vbat and their sources respectively connected to voltage GND via current sources CS 4 and CS 5 . The gates of transistors T 5 and T 4 form the first and second inputs of comparator 12 . Switching means 10 includes two P-channel MOS transistors T 6 and T 7 having their sources connected to the drain of transistor T 2 and their gates respectively connected to the sources of transistors T 4 and T 5 . Transistors T 4 and T 5 form a differential pair. The drains of transistors T 7 and T 6 respectively form the first and second output terminals of switching means 10 . Control means 14 includes an N-channel MOS transistor having its source connected to voltage GND, and having its drain and its gate connected to each other as well as to a current source generating current Iref 1 . The drain and the gate of the transistor of control means 14 form the input terminal of control means 14 . Switch 16 is an N-channel MOS transistor connected as a current mirror with the transistor of control means 14 . The source of transistor 16 is connected to voltage GND and the drain of transistor 16 is connected to node G 3 . Control means 20 includes an N-channel MOS transistor having its source connected to voltage GND and having its drain and its gate connected to each other and to a current source generating current Iref 2 . The drain and the gate of the transistor of control means 20 form the input terminal of control means 20 . Switch 22 is an N-channel MOS transistor connected as a current mirror with the transistor of control means 20 . The source of transistor 22 is connected to voltage GND and the drain of transistor 22 is connected to node G 3 .

When voltage Vout is smaller than voltage Vt, the voltage of the source of transistor T 5 is smaller than the voltage of the source of transistor T 4 . As a result, the voltage of the gate of transistor T 7 is smaller than the voltage of the gate of transistor T 6 . Transistor T 7 is then more conductive than transistor T 6 . Transistors T 4 , T 5 , T 6 , and T 7 are chosen so that current Irep then only runs through transistor T 7 and not through transistor T 6 . The drain of the transistor of control means 14 then receives current Irep. As long as current Irep is smaller than current Iref 1 , no current runs through transistors 14 and 16 . When current Irep becomes greater than current Iref 1 , transistors 14 and 16 are run through by a current Irep−Iref 1 . When current Irep−Iref 1 is high enough, the voltage drop across resistor R 1 turns transistor T 3 on to limit current Iout to a value It 1 , as described in relation with FIGS. 2 and 3.

When voltage Vout is greater than voltage Vt, current Irep runs through transistor T 6 and not through transistor T 7 . The operation of control means 20 and of switch 22 is then similar to the operation of control means 14 and of switch 16 which has just been discussed and current Iout is limited to a value It 2 .

FIG. 5 schematically shows a second embodiment of the voltage regulator of FIG. 2 . Device 8 includes P-channel MOS transistor T 2 , voltage comparator 12 , switching means 10 , switches 16 and 22 and control means 14 and 20 of the previously-described device 8 . A P-channel MOS transistor T 3 ′ has its source connected to voltage Vbat and its drain connected to the gate of transistor T 1 . The gate of transistor T 3 ′ is connected to the output of an operational amplifier 26 supplied between voltages Vbat and GND. Non-inverting and inverting inputs E+ and E− of amplifier 26 are connected to voltage Vbat respectively via impedances Z 1 and Z 2 . Impedances Z 1 and Z 2 are equal and matched, so that any variation in the value of Z 1 , for example due to a temperature or manufacturing process variation, corresponds to an equal variation of Z 2 . The inverting input of amplifier 26 is also connected to voltage GND via a current source generating a predetermined constant current 12 . The non-inverting input of amplifier 26 is connected to voltage GND via switches 16 and 22 , in parallel.

The control of transistor T 3 by amplifier 26 depends on the ratio of the voltage drops in impedances Z 1 and Z 2 . Impedances Z 1 and Z 2 being equal and matched, the control of transistor T 3 ′ is independent from the values of impedances Z 1 and Z 2 and only depends on the ratio between the currents flowing through impedances Z 1 and Z 2 . Current 12 flowing through impedance Z 2 is constant. The current flowing through impedance Z 1 is comparable to the current flowing through resistor R 1 of FIG. 3 . Current Iout thus depends on Iref 1 or Iref 2 according to whether voltage Vout is smaller or greater than voltage Vt. The control of transistor T 3 being independent from values Z 1 and Z 2 , current Iout is independent from the variations of impedances Z 1 and Z 2 , which is an additional advantage of the present invention. Further, the gain of amplifier 26 may be chosen to be high so that the control of transistor T 3 ′ is little responsive to a drift in the threshold voltage of transistor T 3 ′, which is another advantage of the present invention.

When current Iout varies abruptly, the current-limiting loop reacts with a delay, especially introduced by amplifier 26 . This delay can cause the occurrence of a current peak Iout between the time when current Iout starts increasing and the time when transistor T 3 ′ is turned on. A protection block (not shown) may be arranged to turn transistor T 3 ′ unconditionally on for a predetermined duration after any abrupt drop in voltage Vout or upon powering on of the voltage regulator, to suppress such a current peak.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. As an example, the present invention has been described in relation with specific control means 14 and 20 , switches 16 and 22 , voltage comparator 12 , and switching means 10 , but those skilled in the art will easily adapt the present invention to a regulator using elements having a different structure but performing same functions.

The present invention has been described in relation with a voltage regulator using positive voltages Vbat, Vref, and Vt, but those skilled in the art will easily adapt the present invention to a voltage regulator using negative voltages by inverting the types of the described MOS transistors.

The present invention has been described in relation with a voltage regulator in which voltage Vt is chosen to be smaller than voltage Vref, but those skilled in the art will easily adapt the present invention to a voltage regulator using equal voltages Vt and Vref. In this case, the differential pair formed by transistors T 4 and T 5 will be imbalanced to turn transistor T 6 on when Vout=Vref=Vt.

For simplicity, the present invention has been described in relation with a voltage regulator using a non-resistive feedback loop and providing a voltage equal to a received reference voltage Vref. However, those skilled in the art will easily adapt the present invention to a voltage regulator having a feedback loop which includes a resistive bridge, and which provides as an output a voltage different from the received voltage Vref.

The present invention has been described in relation with a voltage regulator using a power transistor T 1 , but those skilled in the art will easily adapt the present invention to a voltage regulator using another type of voltage-controlled power switch.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.